H53J-01 INVITED
Transport processes of nitrogen, phosphorus, and pesticides in five agricultural watersheds in the United States
Transport processes affecting agricultural chemicals in the hydrological cycle were investigated at five watersheds in major agricultural settings, representing both different agricultural practices and climatic settings of the United States. Watersheds were chosen in two semi-arid regions of California and Washington that utilized irrigation; in Nebraska (typical of mid-west corn and soybean rotation); in an Indiana basin with tile drainage; and in a Maryland watershed where ground-water discharge supplied a substantial portion of the annual streamflow. The study design was to provide a mass-balance of water and agricultural chemicals originating from the atmosphere or irrigation water, through the unsaturated zone, along a ground-water flow path, and discharge of the ground water to a stream. Although overland flow associated with storms transported the bulk of nutrient and pesticide compounds to streams in most of these watersheds, ground-water transport, as indicated by flow separation analysis, was also important for annual loads of nitrate and pesticide degradates. Total nitrogen, mainly in the form of nitrate, was the most important nutrient with respect to mass loading in these streams, and pesticide transport was usually greatest during the first few rainfall events following application. In contrast, ground-water transport of pesticide degradates contributed to their mass loading throughout the year in base-flow dominated streams. Although subsurface transport of phosphorus has not been given much attention in previous studies of agricultural chemicals, concentrations were elevated in the unsaturated zone and along ground-water flow paths to streams in some of the watersheds, and contributed to the annual stream load. This was particularly true in the basins of the western United States. For instance, in the Washington basin, discharging ground water accounted for up to 30 percent of the annual phosphorus stream load.
H53J-02 INVITED
Effects of Crop Rotation, N Management, Tillage, and Controlled Drainage on nitrate-N Loss in Drain Flow
Accurate simulation of agricultural management effects on N loss in tile drainage is vitally important for understanding hypoxia in the Gulf of Mexico. An experimental study was initiated in 1978 at Nashua, Iowa of the USA to study long-term effects of tillage, crop rotation, and N management practices on subsurface drainage flow and associated N losses. The Root Zone Water Quality Model (RZWQM) was applied to evaluate management effects (tillage, crop rotation, N application, and controlled drainage) on N loss in drain flow. RZWQM simulated the observed increase in N concentration in drain flow with increasing tillage intensity from NT (no-till) to RT (ridge till) to CP (chisel plow) and to MP (moldboard plow). It also adequately simulated tillage effects on yearly drain flow and yearly N loss in drain flow. On the other hand, RZWQM adequately simulated lower yearly drain flow and lower flow-weighted N concentration in drain flow under CS (corn-soybean) and SC (soybean-corn) than under CC (continuous corn). The model also simulated higher N loss from fertilizer-N applications than from manure-N applications. Applying the newly suggested N management practice for the Midwest of controlled drainage, the model simulated a 30% reduction in drain flow and a 29% decrease in N losses in drain flow under controlled drainage (CD) compared to free drainage (FD). With most of the simulations in reasonably close agreement with observations, we concluded that RZWQM is a promising tool for quantifying the relative effects of tillage, crop rotation, N application, and controlled drainage on N loss in drainage flow. Further improvements on simulated management effects on N mineralization and plant N uptake are needed, however.
H53J-03 INVITED
Vegetated buffer management practice to improve surface water quality
Vegetated buffer best management practices (BMPs) installed in agricultural landscapes have been suggested as promising candidate tactics to reduce erosion and offsite transportation of agrochemicals. A wide range of vegetated buffer management practices have been installed in many areas to reduce agrochemical loss from applied fields, to filter sediments from tailwaters, and to deter their transportation to water bodies. This presentation will focus on reviewing vegetated buffers and their efficacies in reducing agrochemical offsite movements, with a discussion on the major factors influencing BMP efficacy. Percent removal by various BMPs ranged from 16.7 to 100% for sediments, 29 to 98% for nitrogen, 1 to 100% for phosphorus, and 27 to 100% for pesticides, depending on the setting. Preliminary meta-analyses on the data obtained from the literature review showed that vegetated buffers were mostly effective in removing sediment, followed by pesticides and nutrients. BMP efficacy is mainly influenced by buffer width, buffer slope, rainfall and vegetation. As for sediment reduction, the results based on the limited data showed that buffer width and buffer slope are two major factors influencing mitigation efficacy of vegetated buffers. The results also showed that a design with 10-m width and a 9% slope optimizes the sediment trapping capability of vegetated buffers. The meta-analysis results of this study could provide specific recommendations such as buffer width and slope for future vegetated buffer BMP construction to increase soil and water conservation.
H53J-04
Cost-Effective Allocation of Agricultural Best Management Practices
Implementation of conservation programs is perceived as being crucial for restoring and protecting waters and watersheds from nonpoint source pollution. Success of these programs depends to a great extent on planning tools that can assist the watershed management process. Herein, a novel optimization methodology is presented for deriving watershed-scale sediment and nutrient control plans that incorporate multiple, and often conflicting, objectives. The method combines the use of a watershed model (SWAT), representation of best management practices, an economic component, and a genetic algorithm-based spatial search procedure. For a small watershed in Indiana located in the Midwestern portion of the United States, selection and placement of best management practices by optimization was found to be nearly three times more cost-effective than targeting strategies for the same level of protection specified in terms of maximum monthly sediment, phosphorus, and nitrogen loads. Conversely, for the same cost, the optimization plan reduced the maximum monthly loads by a factor of two when compared to the targeting plan. The optimization methodology developed in this paper can facilitate attaining water quality goals at significantly lower costs than commonly used cost-share and targeting strategies.
H53J-05
Groundwater Monitoring of Land Application with Manure, Biosolids, and other Organic Residuals
Regulatory programs frequently require monitoring of first encountered (shallow-most) groundwater for purposes of determining whether an actual or potential, permitted or incidental waste discharge has had or will have a degrading effect on groundwater quality. Traditionally, these programs have focused on monitoring of incidental discharges from industrial sites. Increasingly, sources with an implied groundwater recharge are subject to monitoring requirements. These recharging sources include, for example, land application of municipal, food processing, or animal waste to irrigated cropland. Groundwater monitoring of a recharging source requires a different approach to groundwater monitoring than traditional (incidental source) monitoring programs. Furthermore, the shallow groundwater aquifer targeted for compliance monitoring commonly consists of highly heterogeneous unconsolidated alluvial, fluvial, lacustrine, glacial, or subaeolian sediments of late tertiary or quaternary age. Particularly in arid and semi-arid climates, groundwater is also frequently subject to significant seasonal and interannual groundwater level fluctuations that may exceed ten feet seasonally and several tens of feet within a three- to five-year period. We present a hydrodynamically rigorous approach to designing groundwater monitoring wells for recharging sources under conditions of aquifer heterogeneity and water level fluctuations and present the application of this concept to monitoring confined animal farming operations (CAFOs) with irrigated crops located on alluvial fans with highly fluctuating, deep groundwater table. http://groundwater.ucdavis.edu
H53J-06
Collateral Geochemical Impacts of Agricultural N Enrichment from 1963 to 1985: A Southern Wisconsin Groundwater Depth Profile
In this study, we used CFC age-dating and biogenic N gas measurements to reconstruct the chronology of groundwater N-enrichment within a bedrock aquifer depth profile beneath a south central Wisconsin agricultural landscape. Four geochemical impacts were associated with a steady groundwater N-enrichment trajectory (40 μ mol L-1 yr-1, r2=0.99) between 1963 and 1985. First, NO3 - became prominent as a mobile electron acceptor. Denitrified N accounted for 36% of the total N within the profile. Denitrification appeared to occur in the near water table environment, in hot spots or hot moments, rather than progressively along lengthy groundwater flowpaths. Second, as a byproduct of soil nitrification, N2O entered groundwater via gravity drainage from the soil at a stable (r2=0.99) mole ratio of 0.24% (N2O-N/NO3-N). The gathering of excess N2O in groundwater is a potential concern relative to greenhouse gas emissions and stratospheric ozone depletion after it discharges to surface water. Third, nitrification driven dissolution of dolomitic soil amendments more than doubled the concentrations of major ions (Ca, Mg, HCO3-) in groundwater by 1985 relative to their concentrations prior to agricultural N-enrichment. Finally, dissolved P increased 0.002 mg L-1 yr-1 from approximately 0.02 mg L-1 in 1963 to 0.07 mg L-1 in 1985, in association with rising Ca concentrations (r2=0.67). Nitrification induced mobilization of Ca may have caused a co-release of P from Ca-rich soil surfaces, promoting its loss in gravity drainage. The geochemical changes in groundwater induced by N-enrichment have implications for understanding the current and future quality of our surface waters.
H53J-07
Spatially based management of agricultural phosphorus pollution from diffuse sources: the SCIMAP risk based approach
Pollution of rivers from agricultural phosphorus is recognised as a significant global problem and is a major management challenge as it involves processes that are small in magnitude, distributed over large areas, operating at fine spatial scales and associated with certain land use types when they are well connected to the receiving waters. Whilst some of these processes have been addressed in terms of water quality forecasting models and field measurements, we lack effective tools to prioritise where action should be taken to remediate the diffuse pollution problem. From a management perspective, the required information is on ‘what to do where' rather than absolute values. This change in focus opens up the problem to be considered in a probabilistic / relative framework rather than concentrating on absolute values. The SCIMAP risk management framework is based on the critical source area concept whereby a risk and a connection are required to generate a problem. Treatments of both surface and subsurface hydrological connectivity have been developed. The approach is based on the philosophy that for a point to be considered connected there needs to be a continuous flow path to the receiving water. This information is calculated by simulating the possible flow paths from the source cell to the receiving water and recording the required catchment wetness to allow flow along that route. This algorithm gives information on the ease at which each point in the landscape can export risk along surface and subsurface pathways to the receiving waters. To understand the annual dynamics of the locational diffuse P risk, a temporal risk framework has been developed. This risk framework accounts for land management activies within the agricultural calendar. These events include the application of fertiliser, the P additions from livestock and the offtake of P in crops. Changes to these risks can be made to investigate management options. The SCIMAP risk mapping framework has been applied to 12 catchments in England as part of the DEFRA / Environment Agency's Catchment Sensitive Farming programme. Result from these catchments will be presented. http://www.scimap.org.uk
H53J-08
Occurrence and Distribution of Agricultural Pesticides and Transport Modeling in Surface and Subsurface Environments
The use of a variety of pesticides has increased dramatically during the past decades to improve agricultural efficiency and productivity. However, these agricultural chemicals are often washed to surface waters by runoff and leached through the vadose zone to ground water, thereby polluting waters and threatening human health as well as aquatic and terrestrial ecosystems. It is of particular importance to develop effective modeling tools to assess the induced nonpoint source pollution, to regulate the use of agricultural pesticides, and to circumvent further deterioration in water quality. Different physically-based pesticide transport models, ranging from simple analytical models to semidiscrete and more rigorous numerical models, are discussed. In particular, the effects of use of pesticides on their occurrence and distribution in surface and subsurface environments are examined in this study. A windows-based integrated pesticide transport model (IPTM) is used to simulate three-phase pesticide transport and transformation and quantify spatial and temporal distributions in a coupled canopy-soil system as well as pesticide loading potential to the adjacent surface water through surface runoff and erosion. Five different pesticides (diazinon, 2,4-D acid, DBCP, simazine, and lindane) are examined. It is found that occurrence and distribution of pesticides in the environment are closely related to their use and determined by a series of pesticide transport and transformation processes. The occurrence and use of pesticides follow extremely complex and dynamic patterns that are affected by numerous factors related to their use and properties, hydrology, and agricultural activities. It is also found that changes in pesticide use (application quantity, frequency, timing, and method) may result in distinct environmental fate of pesticides in terms of their occurrence extent as well as spatial and temporal distributions.